Show current selected attributes when re-edit Products by Attribute block

[farookibrahim]

Fixes #3084

To reproduce

Steps to reproduce the behavior:

1. Add a Products by Attribute block.
2. Select some attributes and click on Done.
3. Click on the pencil icon to edit the block again.
4. Notice the attributes you previously selected are not checked by default, so it's easy to forget what was your previous attribute selection.

Expected behavior

When clicking on the edit button, the currently selected attributes should be checked by default, like it happens with the Products by Category block, for example.

How to test the changes in this Pull Request:

1. Add a Products by Attribute block.
2. Select some attributes and click on Done.
3. Click on the pencil icon to edit the block again.
4. Now selected attributes checked by default.

Screenshots:

[senadir]

Hi Farook, Thank you for your contribution! can you please you update:

• The PR title to outline what you fixed
• The PR body to explain what was fixed and how to test those changes (copying from the issue is okay).

[farookibrahim]

Hi @senadir, I updated the title and comment now.

[senadir]

Hi @farookibrahim, sorry for taking so long to get back at this, I tested it and it works as expected.
Thank you for your contribution!